THE SPINES. 653 



serrate and flattened (PL I c .f. 32, ss). As a general thing the actinal inter- 

 ambulacral spines of the Cidaridae are small compared to those of the abac- 

 tinal primary interambulacral tubercles. In the ornamentation of the spines 

 we find extraordinary differences, all possible passages between the per- 

 fectly smooth pointed spines to spiny or lamellar and cup-shaped ornamen- 

 tation or nodes. Compare this passage in the spines of Phyllacanthus 

 baculosa, which pass gradually from the extreme of PL I c . f. 35 to that of 

 PL P.f. -it;", or in the spines of Phyllacanthus verticillata passing from PI. P. 

 f. 42 to PI P.f. 40", and in the gradual transition of the smooth spines of 

 Goniocidaris to the cupped spine so characteristic of the genus. The greatest 

 differences to be noticed are perhaps found in the spines of the huge Phylla- 

 canthus gigantea, where we find at the same time the lamellar and spiny 

 structure so frequently found in the fossil spines of many species of Bhabdo- 

 cidaris. 



The microscopic structure of a section of a primary interambulacral spine 

 shows the cause of this great variety, which is due to the independent 

 growth of the outer sheath ; while in other regular Echini the growth of 

 the outer layer takes place uniformly with the increase in size of the spine. 



In the Perischoechinidae the centre of the shaft is hollow ; the body of the 

 shaft is made up of very close minute cells, with an outer independent 

 sheath of solid lamellar wedges, as in the Cidaridae proper.* 



In the fossil genus Hemicidaris the structure of the spines, as shown by a 

 section, is identical with that of the recent Cidaridae of the type of Cidaris 

 metularia ; the position of this genus among the Cidaridae has been con- 

 sidered as doubtful by Wright ; he has established for it a separate family. 



The other Echini in which the shaft of the spine has a homogeneous cellu- 

 lar structure are the Arbaciadae and the Salenidae. In the Salenidae (PI. 

 XXXV. f. ie) the radiating lines of cells are quite marked, and we have 

 the same independent outer sheath as in the Cidaridae proper. In the Ar- 

 baciadae the structure of the inner part of the shaft is like that of the 

 Cidaridae, only the cells are comparatively larger (PI. XXXV. f. 7, 0), 

 and the marginal wedges are large, elliptical, and disconnected, similar to 

 the ordinary wedges of the Echinidae proper, only developing so as to form 

 a deeply indented edge (PI. XXXV. f. 7, 0) in a transverse section. The 

 homogeneous central part of the shaft is well seen in the longitudinal sec- 

 tions of the Cidaridae (PI. XXXV f. 1, 4) and of Arbacia (PI XXXV. 



* Muller has given figures of spines of Lepidocentrus in his Neue Echinodermen d. Eifel, 1857. 



